<!DOCTYPE html>
<body>

<select id="multiselect" multiple="multiple">
    <option value="option 1">Option 1</option>
    <option value="option 2">Option 2</option>
    <option value="option 3">Option 3</option>
</select>

<script>
var multiselect = document.getElementById('multiselect');
multiselect.focus();

var isMacOSX = navigator.userAgent.indexOf("Mac") != -1;
var ctrlKey  = isMacOSX ? "metaKey" : "ctrlKey";

if (window.eventSender) {
    eventSender.keyDown("ArrowDown");
    eventSender.keyDown("ArrowDown", ctrlKey);
    eventSender.keyDown("ArrowDown", ctrlKey);
}
</script>
</body>
</html>
